Give fields a model class in the service library.

runtime/bin/vmservice/observatory/test/inbound_references_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library inbound_references_test;
 
 import 'dart:async';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
@@ skipping 12 matching lines @@
   n.edge = e;
   array = new List(2);
   array[0] = n;
   array[1] = e;
 }
 
 var tests = [
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
-    Instance e = lib.variables.where((v) => v.name == 'e').single['value'];
+    Instance e = lib.variables.where((v) => v.name == 'e').single.value;
     var id = e.id;
     return isolate.get('/$id/inbound_references?limit=100').then(
         (ServiceMap response) {
           List references = response['references'];
           hasReferenceSuchThat(predicate) {
             expect(references.any(predicate), isTrue);
           }
 
           // Assert e is referenced by at least n, array, and the top-level
           // field e.
-          hasReferenceSuchThat((r) => r['slot'] is Map &&
-                                      r['slot']['type']=='@Field' &&
-                                      r['slot']['name']=='edge' &&
+          hasReferenceSuchThat((r) => r['slot'] is Field &&
+                                      r['slot'].name=='edge' &&
                                       r['source'].isInstance &&
                                       r['source'].clazz.name=='Node');
           hasReferenceSuchThat((r) => r['slot'] == 1 &&
                                       r['source'].isList);
           hasReferenceSuchThat((r) => r['slot']=='<unknown>' &&
-                                      r['source']['type']=='@Field');
+                                      r['source'] is Field);
     });
 }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);